The purpose of this protocol is to determine the underlying genetic defects in patients with possible inherited forms of immunodeficiency. This protocol is used to evaluate patients with many forms of inherited immunodeficiency. In the past, we focused on patients with adenosine deaminase deficiency and purine nucleoside phosphorylase deficiency. We currently use this protocol to screen patients with possible DiGeorge syndrome for later enrollment in protocol 668 (thymus transplantation). This protocol is used to evaluate patients with complete DiGeorge syndrome who are treated with therapies other than thymus transplantation in order to compare those therapies with postnatal thymus transplantation. It is also used for patients with partial DiGeorge syndrome to better understand how their T cell function changes with time. Lastly, candidate disease genes are investigated under this protocol in clinical situations in which a possible new genetic defect might be identified. The methods include cellular immunology techniques (flow cytometry, lymphocyte proliferative studies, T cell receptor rearrangement excision circle quantitation) evaluating T and B cell function and molecular biology techniques (RNA, DNA isolation, cDNA preparation, PCR, cloning, sequencing) to evaluate the sequence of specific genes. The significance of this work is in the better understanding of inherited forms of immunodeficiency.